1. Field of the Invention
The invention relates to a bioreactor for culturing microorganisms and cells.
2. Description of the Related Art
In order to culture microorganisms or cells of animal or plant origin in a bioreactor in high density, it is frequently necessary to keep the concentration of toxic metabolic products such as ammonium or lactate in the culture medium as low as possible. This is achieved by continuously removing cell-free polluted medium from the bioreactor and replacing it by fresh culture medium. The separation of cells and culture medium can be achieved, for example, by centrifugation or filtration. The operation of discharge and replacement with an equal volume of culture medium is designated perfusion. The perfusion step can proceed outside the bioreactor, but this is complex in terms of apparatus and is associated with a high risk of contamination. In the case of bioreactors having flexible walls, cell-free culture medium can be taken off in a very simple manner by mounting a perfusion filter on the inner wall of the reactor.
A disposable perfusion bioreactor of this type is marketed by Sartorius Stedim Biotech GmbH under the name CultiBag® RM. Mixing proceeds here by means of a tilting device which makes the culture medium run constantly to and fro in the flat bag. The cell-free culture medium is separated off via a filtration module in the form of a filter bag consisting of a large-pored filter medium the periphery of which is welded to one of the inner bioreactor walls.
The necessity of selecting a filter medium which is impermeable to cells and readily permeable to other components of the cell suspension limits the effective pore size of the filter layer to a maximum of 10 μm. Suitable filters are microfiltration membranes which are known to have a narrow pore size distribution with a large void volume. A disadvantage of the materials previously used as perfusion membrane in the bags is their inadequate mechanical strength for long-term use.
Disadvantages of this bioreactor result, in addition, from the circumstance that in the case of the filter medium, warping and creasing of the filter medium occurs which, in relatively long-term operation, leads to breaking of the filter medium owing to the mechanical stress in the crease regions which is caused by the movement of the culture medium.
U.S. Pat. No. 6,544,788 B2 discloses a bioreactor having flexible walls which, for the purpose of perfusion, has a filter module floating on the surface of the culture medium. The mechanically stable floating body, however, is of limited use if a reduced culture volume is employed in the starting phase of a cell culture. Then, the float housing can impact onto the bag wall and damage same. In addition, the filter surface areas required for the perfusion are so large that exchange of gases, in particular oxygen, with the culture medium is impaired.
The object of the present invention is therefore to propose a bioreactor having a filter module suitable for perfusion, which filter module is distinguished by a high mechanical stability in long-term use and does not hinder exchange of gases with the culture medium.